In the field of renal dialysis, the use of dialyzer cartridges to remove blood-borne uremic toxins and by-products of metabolism has been conventional for may years. Typically, such a cartridge contains essentially two chambers separated by a semipermeable membrane. Blood is perfused through the first chamber and returned to the patient. A dialysis solution is simultaneously circulated through the second chamber. A concentration gradient is thereby established which causes toxic molecular species contained in the blood to migrate through the semipermeable membrane and into the dialysis solution.
The principle of hemodialysis has been refined extensively. The use of a large plurality of semipermeable hollow fiber membranes in dialyzers is now utilized to greatly increase the surface area to facilitate diffusion across the membrane structure. The hollow fiber membranes are composed of a variety of materials including cellulose acetate, cellulose triacetate, polyacrylonitrile, polysulfone, and regenerated cellulose, the latter being most commonly used. These hollow fibers are small bore capillaries arranged in parallel. The bundle of fibers is typically potted in a curable sealant at both ends.
Upon cure, the embedded fibers are cut through at the ends to expose the bores of the fibers, as disclosed in U.S. Pat. No. 4,227,295 (Bodnar et al). The fiber bundle is then enclosed in a housing which forms the dialysate chamber. Examples of dialyzers of this construction together with the mechanical details as to closure, inlet and outlet ports, and the like are illustrated in U.S. Pat. No. 4,283,284 (Schnell) and U.S. Pat. No. 4,600,512 (Aid).
In the operation of the dialyzer, patient blood is pumped through the hollow fiber bundle, and a dialysis solution is pumped through the dialysate chamber so that dialysis solution constantly bathes the exterior hollow fiber surfaces. Pump assisted movement of blood through the dialyzer is required in order to displace a sufficient volume for effective cleansing within a treatment time of less than six hours. Another pump propels dialysis solution through the dialysate chamber and also regulates by valves under microprocessor control, the proper mixing of dialysis solution concentrate and water. Electrolyte concentrations are monitored by continuous conductivity measurement. Another function of this pumping device is to carefully control fluid back pressure so as to prevent excessive water loss form the blood.
It is also important to control loss of sodium to the dialysis solution during treatment so as to avoid dialysis disequilibrium syndrome, a condition thought to result from a too rapid reduction in salt concentration from about 145 mEq/liter prior to treatment, to 134 mEq/liter after treatment. Various methods of changing the salt content of the dialysis solution during treatment to avoid an abrupt reduction in sodium have been proposed, as in U.S. Pat. No. 4,722,798 (Goss).
Other medically adverse effects upon patients undergoing dialysis may result from an inadvertent failure to completely dialyze the patient. At the present time, the average dialysis patient has a life expectancy of only about five years. One reason these patients tend to have a short life expectancy is the deleterious effect of a chronic buildup of various toxins that either are not eliminated at all, i.e. do not pass through the hollow fibers, or are not completely reduced to nontoxic levels. The identity of many of these supposed toxins is not known, although those species known to be eliminated in urine, such as creatinine, urea, phosphate, hydrogen ions, etc. are associated with serious medical consequences when permitted to accumulate in excess of normal levels.
It is common practice in the field of hemodialysis to reuse dialysis cartridges. There is technology available for cleaning, disinfecting, or sterilizing used dialysis cartridges. (See, for example U.S. Pat. No. 4,695,385). Eventually, however, the cartridge must be discarded because it loses its dialyzing competency. At the present time, the competency of dialyzers is not being rigorously monitored, and a dialyzer is discarded when it visually appears unclean after recleaning, or when fiber bundle volumes and ultrafiltration rates become normal. It is now known that severe dialyzer dysfunction can occur even when appearance, total cell volumes and ultrafiltration rates are normal, as reported by Delmez et al., "Severe dialyzer dysfunction during reuse," Kidney International, 35:244 (1989). It is also known that dialyzer competency can not be accurately predicted by the age of the dialyzer of the number of uses.
One measure of adequacy of dialysis for the individual patient as to a particular dialyzer is calculated from the following equation: EQU KT/V.gtoreq.0.8
wherein V is an expression of the volume of distribution of urea which is approximately equal to total body fluid volume derived for each individual patient from data such as height, weight, and sex, K is the urea clearance of the dialyzer in use in ml of blood totally cleared in urea each minute, and T is the treatment time. A typical product insert accompanying a dialyzer unit contains a graph of urea clearance versus blood flow rate obtained by random testing of a sample of dialyzers from a particular manufacturing lot. Upon incorporating these values into the above equation, the minimum treatment time can be calculated for a given KT/V value. Other parameters that may be varied to achieve adequate dialysis include blood flow rate, dialysis solution flow rate, dialyzer competency, and temperature.
It has been determined empirically that KT/V values of about 0.8 or greater are associated with low levels of morbidity. See Gotch, L A., Sargent, J A., Kidney Int., 28: 526-537, 1985. Even in the use of new dialyzer units there is some risk that a unit selected from a particular lot will have a significantly lower K value than the value depicted by the product insert graph. The patient receiving treatment from such a unit is therefore at risk of being under-dialyzed. The likelihood of under-dialysis increases upon reuse of the dialyzer because of the unpredictability of loss of dialyzer competence.